Bump opencode 1.14.44 -> 1.14.50; cut over to split-base pipeline

- Bump OPENCODE_VERSION 1.14.44 -> 1.14.50 in Dockerfile.variant
- Cut over: docker-publish-split.yml now triggers on push: tags: v*
  (was workflow_dispatch only). RELEASE_TAG and PROMOTE_LATEST derived
  from github.ref_type/ref_name for tag-push; inputs still available
  for manual workflow_dispatch runs.
- Delete docker-publish.yml (retired, replaced by split-base pipeline)
- Delete Dockerfile (retired, replaced by Dockerfile.base + Dockerfile.variant)
- Update CHANGELOG: promote Unreleased -> v1.14.50
- Update AGENTS.md, .gitea/README.md, validate.yml: remove all references
  to the old single-Dockerfile pipeline and WIP migration plan

Dockerfile.variant

@@ -32,7 +32,7 @@ ARG USER_NAME=developer
 
 # ── Install opencode via npm ─────────────────────────────────────────
 ARG INSTALL_OPENCODE=true
-ARG OPENCODE_VERSION=1.14.44
+ARG OPENCODE_VERSION=1.14.50
 RUN if [ "${INSTALL_OPENCODE}" = "true" ]; then \
       NPM_CONFIG_PREFIX=/usr npm install -g opencode-ai@${OPENCODE_VERSION} && \
       opencode --version ; \